Emails NDR with error 'Headers too large'


Your emails are not being delivered and you receive an NDR stating:

----- The following addresses had permanent fatal errors ----- <********@********.com> (reason: 552 5.6.0 Headers too large (32768 max))


Your "Receive" Connector is not configured correctly in Office 365, causing it to retain internal Exchange email headers. 


To fix this issue, carry out one of the following methods listed below: 

  • Use the Signatures for Office 365 Setup Wizard


  •  Configure the connector manually

Use the Signatures for Office 365 Setup Wizard

  1. Remove the Office 365 connectors and Transport Rule associated with Signatures for Office 365.
  2. Log into the Exclaimer Cloud portal and click the 'Settings' button located in the top right of your screen:

    Settings button in Signatures for Office 365.

  3. A separate lightbox will open. Navigate to the Office 365 Connector Setup section at the bottom and click 'Setup':

    Office 365 Connector Setup.

  4. Enter the credentials for your Global or Exchange administrator account, click 'Next' and follow the steps shown in the Wizard:

    Global admin credentials.

  5. When the Wizard is complete, send a test email with the same variables as the previously bounced email.

If you do not wish to use the Signatures for Office 365 Setup Wizard, you can update your "Receive" connector manually by following the steps below:

Configure the connector manually

Note: DO NOT continue if you have already gone through the Setup Wizard as above.

  1.  Sign into your 'Office 365 admin center', go to the 'ADMIN' option and select 'Exchange':

    Navigate to Exchange in the Office 365 admin center.

  2. Click 'Connectors' under 'Mail Flow':

    Open Office 365 connectors.

  3. Select your "Receive" connector and click the 'Edit' icon. If you are not sure which connector to choose, it will be the one that is set to send email from 'Your organization's email server' to 'Office 365':

    Edit your
  4. Uncheck the 'Retain internal Exchange email headers (recommended)' tickbox:

    Uncheck 'Retain internal Exchange email headers' tickbox.

  5. Save the connector.

You will also want to check the smart host details in the "Send" connector are correct.

  1.  Select the "Send" connector and click the 'Edit' icon:

    Edit the 'Send' connector in Office 365.

  2. Navigate to the section 'How should Office 365 connect to your email server?'. In the field located under 'And the subject name or subject alternative (SAN) matches this domain name:', make sure the certificate has been input correctly. For example, if you are based in Europe, it should read as per below:

    Certificate name needs to be input coreectly.

  3. Save the connector.

Now, send a test email with the same variables as the previously bounced email.

Please be aware that any changes to a Transport Rule may take up to an hour to become operational. If you find that the email signature is not added to the test email, wait a short time and send it again.

Haven't found a solution to your problem?

Please click the button below, login to the Exclaimer Cloud portal and raise a support ticket. A member of the Exclaimer Support team will then be in touch.

Raise a support ticket